ROOM-TEMPERATURE SYNTHESIS OF SILICATE MESOPOROUS MCM-41-TYPE MATERIALS - INFLUENCE OF THE SYNTHESIS PH ON THE POROSITY OF THE MATERIALS OBTAINED

Pure silica MCM-41 materials were synthesized at room temperature at d ifferent pH values ranging from 8.5 to 12. The solids were characteriz ed by XRD, Si-29-MAS-NMR spectroscopy and N-2 and Ar adsorption measur ements. When the synthesis pH is too high, the degree of polycondensat ion of the silicate species is too low and, after removal of the organ ic species by calcination, amorphization occurs. For an intermediate p H value (11), the N-2 and Ar adsorption isotherms at 77 K, determined on the corresponding solid, are 'type I-b like' and are characteristic of the presence of primary and secondary micropores (d<2 nm). At a lo w pH value (8.5), a typical MCM-41 material with a mesopore diameter o f similar to 2.6 nm is obtained. The adsorption isotherms are of type IV. The influence of the thermal treatment on the as-synthesized sampl es was studied, and an effect was observed in the material containing micropores. Whereas a short calcination time at 873 K leads to a decre ase in the intensity of the XRD peaks, a prolonged calcination time at this temperature leads to an improvement in the degree of organizatio n and to an increase in the total microporous volume of about 18%. Thi s increase can be explained by the greater homogeneity of the micropor e surface after such a thermal treatment. (C) 1997 Elsevier Science B. V.